Power transmission belts having enhanced properties

ABSTRACT

The present invention is directed to the incorporation of of functionalized polyethylenes, in amount of about 4% to 50% by weight based upon the weight of the elastomer, into ethylene alpha olefin elastomers, such as EPDM elastomer compositions, which are crosslinked by peroxides, which results in improved properties, such as hardness and modulus of elongation, and can result in improved higher abrasion resistance, wear resistance, coefficient of friction, tensile strength, and high temperature properties, which are beneficial to power transmission products, such as power transmission belts.

BACKGROUND OF THE INVENTION

The present invention is directed to improved ethylene-alpha-olefinelastomers, especially EPDM-based elastomers, for use in manufacturingpower transmission belts. More specifically, the invention is directedto such elastomers which incorporate certain types of functionalizedpolyethylenes will result in belts having enhanced physical properties.

Power transmission belts are known. See for example, U.S. Pat. No.6,561,937 (Wegele); U.S. Pat. No. 6,695,734 (Hedberg et al); U.S. Pat.No. 5,610,217 (Yamell et al) and U.S. Pat. No. 6,251,977 (George et al).Proposals have already been made on numerous occasions to use EPDM(ethylene propylene diene terpolymer) elastomers cured by organicperoxides in the manufacture of power transmission belts, because of thequalities and advantages of such elastomers such as cost, operatingtemperature range, and ability to withstand oxygen and ozone, suchEPDM-based elastomers having additives which improve their dynamicproperties such as resistance to fatigue and to wear, their breakingstrength, and their modulus of elasticity, and also their adhesion totraction cords, which additives are generally constituted by metallicsalts of α,β-unsaturated organic acids (in particular zinc methacrylate)plus reinforcing fillers such as carbon black and possibly fibers, e.g.aramid fibers. For example, the Yarnell '217 patent teaches the use ofα,β-unsaturated organic acids in peroxide cured EPDM elastomers. TheGeorge et al '977 patent teaches the inclusion, in peroxide curedelastomers, of an elastomer grafted with maleic anhydride which reactswith the α,β-unsaturated organic acid metallic salt included in thecomposition to reinforce the curing of the EPDM-based elastomer and toimprove its dynamic characteristics, such as in particular its modulusof elasticity, its breaking strength, and its hardness. The elastomergrafted with maleic anhydride can be a polybutadiene, polyisoprene,polypropylene, or an ethyl vinyl acetate (EVA) copolymer, and theelastomer is used as a power transmission belt.

Oxidized polyethylenes are known and are taught by U.S. Pat. Nos.2,683,141 and 3,060,163 to Erchak. They are used to form stable,translucent emulsions for use in floor waxes, in coatings for asbestosshingles, paper and textiles, and in inks for application to varioussurfaces. They have been incorporated into EPDM compositions, such asthose described in U.S. Pat. No. 4,990,568 to Benefield et al. InBenefield the EPDM is admixed with an oxidized or carboxylatedpolyolefin, in at an amount of 2 to 20% of the elastomer, to improve theproblem of adherence of coatings applied to the surfaces of articlesformed from the thermoplastic elastomers.

SUMMARY OF THE INVENTION

The present invention is the result of the discovery that theincorporation of functionalized polyethylenes into ethylene alpha olefinelastomers, such as EPDM elastomer compositions, which are cured byperoxides, will result in improved properties, such as hardness andmodulus of elongation, and can result in improved higher abrasionresistance, wear resistance, coefficient of friction, tensile strength,and high temperature properties. Enhancement of these properties isbeneficial to power transmission products, such as power transmissionbelts. Further, the formulation results in a reduction in the viscosityof the uncured compound, and results in easier processing conditions.

The incorporation of the functionalized polyethylene is at an amount ofabout 4% to 50% by weight based upon the weight of the elastomer. Whenthe composition is cured by the peroxide that is included in themixture, the functionalized polyethylene is also crosslinked into thecomposition, and the result is an improved belting composition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a differential scanning calorimeter graph of the exotherms ofuncured versus cured compounds of a composition containingfunctionalized polyethylene;

FIG. 2 is a differential scanning calorimeter graph, similar to FIG. 1,of the exotherms of uncured versus cured compounds of a compositioncontaining another functionalized polyethylene;

DETAILED DESCRIPTION OF THE INVENTION

The structures of power transmission belts are well know and illustratedin patents such as U.S. Pat. Nos. 6,251,977, 6,561,937, and 5,610,217,the disclosures of which are incorporated herein by reference. The beltis designed to rotate any rotary member, and can be of the “poly-V”type. As such it would have an inside surface and a certain number ofcircumferential ribs of trapezoidal cross-section, where the ribs aremutually parallel and extend over the entire length of the belt and aredesigned to be engaged in grooves of complementary shape in the pulleyson which the belt is mounted.

The belt has at least one sheet of traction cords which are embedded inthe ethylene-alpha-olefin elastomer of the belt, between its top surfaceand the ribs, with the cords being spiral-wound inside the belt and withthe number of turns thereof being a function of the mechanicalcharacteristics desired of the belt.

The ethylene-alpha-olefin elastomers useful in the present inventioninclude but are not limited to copolymers composed of ethylene andpropylene units (EPM), ethylene and butene units, ethylene and penteneunits, or ethylene and octene units (EOM), and terpolymers composed ofethylene and propylene units and an unsaturated component (EPDM), aswell as mixtures thereof. As the unsaturated component of EPDM, anyappropriate non-conjugated diene may be used, including for example,1,4-hexadiene, dicyclopentadiene or ethylidenenorbornene (ENB). Theethylene-alpha-olefin elastomer preferred in the present inventioncontains from about 35% by weight to about 80% by weight of the ethyleneunit, from about 65% by weight to about 25% by weight of the propyleneor octene unit, and 0-10% by weight of the unsaturated component. In amore preferred embodiment, the ethylene-alpha-olefin elastomer containsfrom about 55% to about 78% by weight of the ethylene unit, and in amost preferred embodiment, the ethylene-alpha-olefin elastomer containsfrom about 65% to about 75% of the ethylene unit. At these morepreferred ethylene unit content levels, endless belts incorporating astheir main belt body portions the ethylene-alpha-olefin elastomiccompositions of this preferred embodiment of the present inventionexhibit improved pilling resistance. The most preferredethylene-alpha-olefin elastomer is EPDM.

To form the elastomer composition of the present invention theethylene-alpha-olefin elastomer may optionally be blended with less than50% by weight, more preferably up to about 25%, and most preferably fromabout 5% to about 10% based on the total elastomeric content of thecomposition of a second elastomeric material including but not limitedto silicone rubber, polychloroprene, epichlorohydrin, hydrogenatednitrile butadiene rubber, natural rubber, ethylene-vinyl-acetatecopolymer, ethylene methacrylate copolymers and terpolymers, styrenebutadiene rubber, nitrile rubber, chlorinated polyethylene,chlorosulfonated polyethylene, alkylated chlorosulfonated polyethylene,transpolyoctenamer, polyacrylic rubbers, butadiene rubber, and mixturesthereof, to fine-tune certain mechanical properties such as hightemperature performance and tack.

The elastomer may incorporate metal salts of α,β-unsaturated organicacids. The metal salts of α.,β-unsaturated organic acids that can beuseful in the present invention are metal salts of acids such as forexample, acrylic, methacrylic, maleic, fumaric, ethacrylic,vinyl-acrylic, itaconic, methyl itaconic, aconitic, methyl aconitic,crotonic, alpha-methylcrotonic, cinnamic, and 2,4-dihydroxy cinnamicacids. These salts may be of zinc, cadmium, calcium, magnesium, sodiumor aluminum, and are preferably those of zinc. The preferred metal saltsof α.,β-unsaturated organic acids are zinc diacrylate and zincdimethacrylate. The most preferred metal salt of unsaturated organicacid is zinc dimethacrylate. Amounts of the metal salt useful in thepresent invention may range from about 1 to about 30 phr, and arepreferably from about 5 to about 20 phr. In the most preferredembodiment, the metal salt is zinc dimethacrylate used in an amount ofabout 5 phr when used in conjunction with EPDM mixed with up to about10% of silicone rubber, and from about 10 to about 20 phr and morepreferably about 15 phr when used in conjunction with the otherethylene-alpha-olefin elastomers useful in the present invention.

The functionalized polyethylenes that can be employed in the presentinvention include oxidized polyethylenes and copolymers of polyethylene,such as ethylene maleic anhydride and ethylene-vinyl acetate copolymers,which will crosslink with the elastomer to increase the hardness andmodulus of the elastomer, which in turn, leads to improved beltproperties, especially for power transmission belts. The preferredfunctionalized polyethylenes include oxidized polyethylene,ethylene-vinyl acetate copolymer, and ethylene maleic anhydridecopolymer. Functionalized polyethylenes are available from HoneywellInternational Inc. under the brand name A-C polyethylene, including A-C307a and 395A, which are oxidized polyethylenes, A-C 400A, which is anethylene-vinyl acetate copolymer, and A-C 575A, which is anethylene-maleic anhydride copolymer.

Oxidized polyethylene wax materials suitable for use in the practice ofthis invention are described in U.S. Pat. Nos. 2,683,141 and 3,060,163,which are incorporated herein by reference. According to the latterpatent, normally solid, hard, waxy polymers of ethylene having anaverage molecular weight between about 1,000 and 3,000 are subjected, inthe liquid phase, to the action of an oxygen-containing gas to causereaction of between 2-17 pounds of oxygen per 100 pounds of wax, i.e. toprovide an oxidized polyethylene wax containing at least 1 percent andpreferably 1-8 percent by weight of oxygen, and acid numbers of not morethan about 50, and preferably between 10 and 45.

The oxidized polyethylene is characterized by having a minimum numberaverage molecular weight above 1000 and preferably at least about 1200,as determined by high temperature vapor pressure osmometry, containingbetween 1-5 percent by weight of total oxygen, and having an acid numberof from 10 to about 35. The described oxidized polyethylene is obtainedby oxidation of polyethylene in molten or finely divided solid form,with free oxygen containing gas, usually air, generally at elevatedtemperature, until the desired oxygen content is obtained. Startingmaterials for making the oxidized polyethylene suitable for use in thepractice of this invention include low molecular weight, low density orlinear low density polyethylene waxes having densities in the range ofabout 0.91 to about 0.96 as, for example, prepared by the processdescribed in U.S. Pat. No. 2,683,141, as well as high density, linearpolyethylene as, for example, prepared in the presence of such well knowcatalysts as the “Phillips” or “Ziegler” type catalysts, havingdensities in the range of about 0.93-0.97 or above. The low molecularweight, low density polyethylene starting material can be oxidized bycontacting in the molten state with a stream of air until the desiredoxygen content has been obtained. The high density, linear polyethylenestarting material is usually oxidized by contact, preferably in thefinely divided solid state, with free oxygen-containing gas, usuallyair, at temperatures ranging from 100° C. up to, but not including, thecrystalline melting point of the polyethylene, until the desired oxygencontent has been obtained.

The ethylene-alpha-olefin elastomeric compositions useful in the endlessbelts of the present invention further comprise from about 25 to about250 phr and preferably from about 25 to about 100 phr of a reinforcingfiller such as carbon black, calcium carbonate, talc, clay or hydratedsilica, or mixtures of the foregoing. The incorporation of from 1 to 30phr of a metal salt of an unsaturated organic acid and from about 25 toabout 250 phr and preferably about 25 to about 100 phr of reinforcingfiller in the peroxide-cured ethylene-alpha-olefin elastomericcomposition preserves the heat stability of conventional peroxide-curedelastomers, while providing the tear strength and dynamic propertiesusually associated with sulfur cured elastomers.

The free-radical producing curatives useful in the present invention arethose suitable for curing ethylene-alpha-olefin elastomers and includefor example, organic peroxides and ionizing radiation. The preferredcurative is an organic peroxide, including but not limited to dicumylperoxide, bis-(t-butyl peroxy-diisopropyl benzene, t-butyl perbenzoate,di-t-butyl peroxide, 2,5-dimethyl-2,5-di-t-butylperoxyhexane,alpha.-alpha.-bis(t-butylperoxy) diisopropylbenzene. The preferredorganic peroxide curative is alpha.-alpha.-bis(t-butylperoxy)diisopropylbenzene. Cure-effective amounts of organic peroxide forpurposes of the present invention are typically from about 2 to about 10phr. Preferred levels of organic peroxide are from about 4 to about 6phr. Sulfur may optionally be added to the organic peroxide curative aspart of a mixed cure system in an amount of from about 0.01 to about 1.0phr, to improve the cured elastomer's Young's modulus without negativelyaffecting its tear resistance.

Other conventional ethylene-alpha-olefin elastomer additives, processand extender oils, antioxidants, waxes, pigments, plasticizers,softeners and the like may be added according to common rubberprocessing practice without departing from the present invention. Forexample, in a preferred embodiment of the present invention, theelastomeric composition also contains from about 0.5 to about 1.5 phr ofan antiozonant or antioxidant and from about 5 to about 15 phr of aparaffinic petroleum oil plasticizer/softener.

The ethylene-alpha-olefin elastomeric compositions useful in the presentinvention may be prepared by any conventional procedure such as forexample, by mixing and milling the ingredients in an internal mixer ormill, such as a two stage Banbury mixer. It should be noted that inblending some of the functionalized polyethylenes, attainingtemperatures above the melting point of the polyethylene grade may benecessary to achieve processing of the mixed materials, and thus auniform blend. But, this is not believed to be necessary for allmaterials.

Typically, a belt made out of an EPDM elastomer of the invention has thefollowing composition prior to curing:

-   -   EPDM: 100 parts by weight;    -   functionalized polyethylene: 5 to 100 parts by weight        (preferable: 20 to 80);    -   carbon black: 10 to 100 parts by weight (preferable: 50 to 60);    -   electrically conductive carbon black: 1 to 10 parts by weight        (preferable: 5 to 6);    -   antioxidant: 0.5 to 8 parts by weight (preferable: 1.5 to 3);    -   organic peroxide: 0.5 to 15 parts by weight (preferable: 2 to        10);    -   curing coagent: 0.5 to 10 parts by weight (1 to 8);    -   metal salts of {acute over (α)},β-unsaturated organic acids        (optional): 2 to 25 parts by weight;    -   plasticizer: 1 to 20 parts by weight (preferable: 1 to 10).

The composition of the elastomer of the invention may also be blendedwith:

-   -   hydrogenated nitrile butadiene rubber (HNBR) constituting 2 to        20 parts by weight approximately to improve resistance to oils        and to solvents, or nitrile butadiene rubber (NBR) in the same        quantities and for the same purpose;    -   polybutadiene (BR) constituting 2 to 20 parts by weight        approximately to improve dynamic properties and resistance to        abrasion;    -   chloro-sulfonated polyethylene with alkyl groups (ACSM)        constituting 2 to 40 parts by weight approximately to increase        resistance to oils and resistance to tearing; and    -   natural rubber comprising 2 to 20 parts by weight approximately        to improve raw adhesion, so that the composition of the        invention then has 98 to 80 parts by weight of EPDM.

In addition, it may also have polyamide, aramid, polyester, rayon,cotton, or glass fibers constituting 3 to 30 parts by weightapproximately to improve the transverse strength of the beltmanufactured with the elastomer composition.

EXAMPLE

In order to demonstrate the present invention, belt compositions weremade and tested. The following composition was mixed and compounded in atwo stage Banbury mixer in a manner typical for rubber compounding:

-   -   EPDM: 100 parts by weight;    -   functionalized polyethylene: 10% by weight;    -   carbon black: 50 parts by weight;    -   electrically conductive carbon black: 3 to 4 parts by weight;    -   antioxidant: 1.5 to 3 parts by weight; and    -   dicumyl peroxide: 16.67 parts by weight

The compositions were shaped into test specimens or sheets, cured atabout 340° F., for about 30 minutes, and evaluated using a RPA 2000rubber process analyzer. The results are set forth in Tables I and II.TABLE I RPA 2000 Rubber Process Analyzer Mettler “Cure” Data Ex-Functionalized Drop MaxS′ − ample Polyethylene sg Point Min S′ Max S′Min S′ No. Employed (g/cc) (° F.) (dNm) (dNm) (dNm) 1 None 3.266 34.9631.70 2 A-C 307A 0.98 284 2.32 31.76 29.44 Oxidized Polyethylene 3 A-C395A 1.00 279 2.33 25.31 22.98 Oxidized Polyethlene 4 A-C 575A 0.92 2232.239 27.11 24.87 Ethylene- Maleic Anhydride CopolymerNote:A-C polyethylenes & copolymers are available from HoneywellInternational, Inc.

TABLE II A-C 575A A-C 307A TEST CONTROL Example Example Tensile Strength1460.3 1392.8 1667.8 (MPa) Elongation (%) 141 175 156 Modulus (MPa)  10%64.9 116.5 203.9  25% 138.7 215.5 343.2  50% 246.2 322.3 492.7 100%712.9 609.3 903.8 141% 1460.3 156% 1667.8 175% 1392.8 Hardness (Shore A)66 73 80

As can be seen from the data above and the DSC (differential scanningcalorimeter) scans shown in FIGS. 1 and 2, the functionalizedpolyethylene is crosslinked with the EPDM and the resulting compositionprovides improved properties for the belting material. The DSC scans ofuncured A-C 307A compound, FIG. 1, and A-C 575A compound, FIG. 2, showmelt point peaks for the functionalized polyethylenes at 127.97° C. and93.59° C. respectively, along with the DiCup cure exotherm at 185-186°C. The same melt point peaks are absent from the DSC scans of curedsamples of these compounds and show that the functionalizedpolyethylenes are crosslinked into the compounds on cure.

The foregoing embodiments of the present invention have been presentedfor the purposes of illustration and description. These descriptions andembodiments are not intended to be exhaustive or to limit the inventionto the precise form disclosed, and obviously many modifications andvariations are possible in light of the above disclosure. Theembodiments were chosen and described in order to best explain theprinciple of the invention and its practical applications to therebyenable others skilled in the art to best utilize the invention in itsvarious embodiments and with various modifications as are suited to theparticular use contemplated. It is intended that the invention bedefined by the following claims.

1. An organic peroxide curable elastomer composition comprising: (a)ethylene-alpha-olefin elastomer, (b) an organic peroxide curing agent,and (c) 4 to 50% by weight, based upon the weight of theethylene-alpha-olefin elastomer, of a functionalized polyethylene. 2.The elastomer composition of claim 1 wherein ethylene-alpha-olefinelastomer is ethylene-propylene-diene terpolymer.
 3. The elastomercomposition of claim 1 wherein the functionalized polyethylene iscrosslinkable with the ethylene-alpha-olefin elastomer.
 4. The elastomercomposition of claim 1 wherein the functionalized polyethylene is anoxidized polyethylene.
 5. The elastomer composition of claim 1 whereinthe functionalized polyethylene is a copolymer of polyethylene andmaleic anhydride.
 6. The elastomer composition of claim 1 whereinwherein the functionalized polyethylene is present in an amount of 5 to100 parts by weight.
 7. The elastomer composition of claim 1 whereinwherein the functionalized polyethylene is present in an amount of 20 to80 parts by weight.
 8. The elastomer composition of claim 1 wherein thecured compound is in the form of a power transmission belt.
 9. Theelastomer composition of claim 1 wherein the cured compound is in theform of a power transmission belt selected from the group consisting ofsynchronous belts, v-belts, and multi-V-ribbed belts.
 10. The elastomercomposition of claim 1 wherein the composition further includes afiller.
 11. The elastomer composition of claim 1 wherein the compositionfurther includes a reinforcing fiber.
 12. The elastomer composition ofclaim 1 wherein the composition further includes a salt of anα,β-unsaturated organic acid.
 13. The elastomer composition of claim 12wherein the salt of an α,β-unsaturated organic acid is zinc diacrylate.14. The elastomer composition of claim 1 wherein the elastomer is ablend of elastomers.
 15. The elastomer composition of claim 1 whereinthe uncured composition comprises the following: (a) 100 parts by weightof ethylene-alpha-olefin elastomer, (b) 5 to 100 parts by weight of afunctionalized polyethylene which is crosslinkable with the elastomer,and (c) 0.5 to 10 parts by weight of an organic peroxide curing agent.